This invention relates to a fixing device for fixing a toner image on a paper sheet to a surface of the paper sheet by heat and mechanical pressure, and more particularly to a fixing device suitable to fix toner images on a paper sheet to opposite surfaces of the paper sheet.
Generally in an apparatus such as an electrophotographic apparatus which prints a toner image on a paper sheet, a paper sheet having a toner image attached thereto is passed through a fixing device in order to fix the toner image to the surface of the paper sheet because otherwise if a toner image is attached to the paper sheet, mere rubbing of the surface of the paper sheet may take toner powder off the surface of the paper sheet, which will not allow practical use of the recorded material. Such a fixing device normally includes a heat roll which is heated to a high temperature by a heat source contained in the fixing device, and a pressure roll for contacting under pressure with the heat roll. Thus, when a paper sheet is passed between the heat roll and the pressure roll in such a manner that an unfixed toner image on a surface of the paper sheet may face the heat roll, the toner image is fixed to the surface of the paper sheet by heat and mechanical pressure of the rolls.
Where a fixing device is provided for each of two different paper paths formed in a printer such as a double-sided printer for printing on opposite surfaces of a paper sheet, conventionally a heat roll and a pressure roll are provided for each of the paper paths.
However, if a pair of rolls is provided for each of two paper paths in this manner, up to two fixing heat rolls are required for a printer. Because a heat roll itself is expensive compared with a pressure roll, employment of two heat rolls in a single printer will make the entire printer expensive.